In the manufacturing process of a semiconductor device, there exists a need to polish silicon nitride, which is chemically unreactive, at a high rate of speed. Most of polishing compositions that have been conventionally used to polish silicon nitride contain abrasive grains and an acid. For example, Patent Document 1 discloses a polishing composition that contains phosphoric acid or a phosphoric acid derivative. Patent Document 2 discloses a polishing composition that contains colloidal silica and an organic acid having a sulfonate group or a phosphonate group and has a pH of 2.5 to 5. These conventional polishing compositions, however, do not adequately satisfy users' needs for the rate of polishing silicon nitride.